1. Field of the Invention
The present invention relates a thermal insulator and, more particularly, to a thermal insulator provided to a heat treatment apparatus used for a semiconductor manufacturing apparatus and a heat recycle system using such a thermal insulator.
2. Description of the Related Art
Since a temperature of a heat treatment furnace, which applies a heat treatment to a semiconductor wafer, becomes very high, a thermal insulator is provided around the heat treatment furnace. That is, heat released from an electric heater for heating the heat treatment furnace is shielded by the thermal insulator so as to prevent the heat from leaking outside of the heat treatment furnace.
Conventionally, such kind of thermal insulator is formed of so-called ceramics wool. The ceramics wool is made of fine fibers of minerals, and the ceramics wool is provided around the heat treatment furnace in the form of a fabric or a board. The thermal insulation of the ceramics wool is achieved by a very low thermal conductivity of minerals, which are materials of the ceramics wool, and tiny spaces formed between the fibers.
Japanese Laid-Open Patent Application NO. 60-80077 discloses a method of insulating a furnace by constituting outer walls of the furnace using a heat insulation member having a honeycomb structure. In this method, an airflow passage is formed by internal spaces of the honeycomb structure member. The furnace is insulated and cooled by passing a cooling air through the airflow passage. Moreover, it is suggested to collect the air used for cooling and store the heat of the air in a thermal storage apparatus or use the collected high-temperature as a combustion air for the furnace.
A heat treatment apparatus of a semiconductor manufacturing apparatus has a structure in which a heat treatment furnace and a conveyance mechanism for conveying semiconductor wafers are provided inside a housing. Therefore, when a heat-treated semiconductor wafers are taken out of the heat treatment furnace, the housing of the heat treatment apparatus is heated. For example, if a heat treatment temperature of the semiconductor wafers is 1000° C., the semiconductor wafers after the heat treatment will be taken out of the heat treatment furnace at a temperature of about 800° C. Therefore, the housing of the heat treatment apparatus is heated by the hot air exhausted from the heat treatment furnace together with the semiconductor wafers. Moreover, the housing is partially heated by the radiation heat of the semiconductor wafers taken out of the furnace.
As mentioned above, the thermal insulator, which insulates the circumference of the heat treatment furnace of the heat treatment apparatus, is formed of a material such as ceramics wool, and if the heat treatment furnace is covered by the thermal insulator having a uniform thickness, any portion of the heat treatment furnace will be provided with uniform heat insulation efficiency. A vertical furnace, which is widely used from among heat treatment furnaces, has a vertical length as long as more than 1 meter. If such a vertical furnace is uniformly heated in the vertical direction by an electric heater, variation in the temperature may occur in the vertical direction of the furnace. That is, since the heated air moves upward within the vertical furnace, a temperature of an upper portion becomes higher than a temperature of a lower portion of the vertical furnace. In order to apply a uniform heat treatment to the semiconductor wafers provided in the vertical furnace, such a variation in the temperature must be eliminated as much as possible.
Then, in a conventional vertical furnace, a power supplied to the electric heater is controlled so that an amount of heat generated by the electric heater in an upper portion of the vertical furnace is larger than an amount of heat generated in a lower portion of the furnace. That is, the power supplied to the electric heater is increased toward a bottom of the vertical furnace. Generally, an electric heater is located in the vicinity of an inner wall of an insulator, which surrounds the vertical furnace. If the thickness of the insulator is uniform, that is, if the insulation efficiency of the insulator is uniform, there is a problem in that the heat of a lower portion of the vertical furnace passing through the insulator and released to the atmosphere is larger than the heat of an upper portion of the vertical furnace passing through the insulator and released to the atmosphere.
In the conventional thermal insulator using ceramics wool, in order to change the heat insulation characteristic, only a control, which merely changes a thickness of the thermal insulator, can be performed and a fine control cannot be achieved. On the other hand, the heat insulation characteristic of the honeycomb structure thermal insulator disclosed in the above-mentioned Japanese Laid-Open Patent Application No. 60-80077 can be changed by controlling a quantity of air flowing through inside of the thermal insulator. However, the heat insulation characteristic can be merely changed with respect to the entire honeycomb structure thermal insulator, and the heat insulation characteristic cannot be changed partially.
Moreover, in the thermal insulator using ceramics wool, a heat treatment furnace cannot be cooled forcibly. Therefore, in order to lower the temperature of the furnace after completion of a heat treatment so as to take out semiconductor wafers from the furnace, it cannot but depend only on cooling by exhausting air in the heat treatment apparatus. For example, in order to lower the temperature of a 1000° C. semiconductor wafer to 800° C., the semiconductor wafer after the heat treatment must be remain inside the heat treatment furnace for a long time. Therefore, there is a problem in that the heat treatment process time of a semiconductor wafer is long.
Moreover, generally a housing of a heat treatment apparatus is formed with a steel plate or the like. If a high-temperature semiconductor wafer is taken out of a vertical furnace within a heat treatment apparatus, a portion of a housing near the semiconductor wafer is heated by radiation. Thus, the heat in the heat treatment apparatus is released to a clean room through the housing, thereby increasing a temperature inside the clean room. For this reason, a load is applied to the air-conditioner for maintaining the clean room air at a constant temperature, and a running cost of the clean room increases. Therefore, in order to prevent a heat generated within a heat treatment apparatus from being released to the clean room through the housing, the housing itself is formed by a thermal insulator and heat insulation efficiency is increased partially.